The polymerization of many common monomers and polymer cross-linking can be induced by exposure to radiation in the form of either photons or electrically charged particles. Energy deposited in the monomer by either radiation is believed to cause formation of free radicals, which in turn can induce polymerization and cross-linking. The term “beam radiation” is used herein to refer collectively to any form of charged particle-beam or photon irradiation that is capable of initiating or otherwise promoting polymerization of a monomer or cross-linking of any other polymer precursor.
Beam radiation is widely used in industrial practice to promote polymerization of monomers or curing of other polymeric coating precursors. Beam radiation typically is derived from a radiation source, and readily lends itself to in-line, continuous processes, such as those appointed for producing indefinite lengths of thin sheet material that include a polymeric coating. For example, the production of such material may include steps of applying the coating precursor to an advancing web and then exposing the coated web to a suitable radiation source. Ideally, the energy of the particles or photons must be sufficient both to penetrate the desired coating thickness and deposit enough energy to generate free radicals. Typically, energetic electrons (often termed “e-beam radiation”) or photons in the ultraviolet (UV) range are employed. A relatively short-duration exposure to radiation of suitable intensity generally suffices, without unduly increasing the substrate temperature. Sources capable of producing any of the foregoing forms of radiation are known in the art.
However, beam radiation by its very nature is effective only for initiating curing of precursor material that lies in a line of sight. That is to say, beams of either charged particles or photons typically emanate from the radiation source and propagate therefrom along a straight-line path. Curing can be induced only for material positioned so as to intercept the direct beam. Although e-beams can in principle be deflected by electrostatic or magnetic forces, in practice the extent of deflection attainable with practical electromagnetic structures is relatively limited. UV light can be directed to some extent by optical structures such as lenses, mirrors, and gratings analogous to those used with visible light. However, UV optics typically are more difficult to construct and maintain than their visible-spectrum counterparts.
Thus, the use of these forms of beam radiation to polymerize and cure polymer precursors that coat simple, planar substrate structures is straightforward. However, beam-induced curing of precursors coating structures that depart from strict planarity is less satisfactory because of the problem of shadowing. More specifically, areas of the substrate that do not lie in the line of sight of the beam source inherently do not receive any radiation, and so may be said to be shadowed. Even if the beam has relatively high divergence and may emanate from a source that is other than a point source (such as a line or other extended source) or that is otherwise diffused, the fundamental limitation of line of sight remains. Thus, the polymerization and cross-linking reactions in shadowed areas cannot be initiated by the beam radiation.
Failure to cure even a small fraction of the precursor in a coating can, in some cases, be highly objectionable. Many uncured monomers commonly used in coatings, notably acrylates, are known to be toxic, to emit objectionable odors, and to impart undesirable tackiness and dust pickup to a surface, even in relatively small amounts. The presence of tacky monomer on a sheet surface makes it difficult to unroll material from a supply roll. Thus, techniques that result in substantially complete curing of a coating to mitigate these detrimental consequences remain highly sought.
The problem of shadowing arises in principle for beam-based curing of the coating of any non-planar article. An approach to the problem of shadowing in curing acrylate coatings has been proposed by Studer et al., Progress in Organic Coatings (2005), 53(2), 126-133; Progress in Organic Coatings (2005), 53(2), 134-146; and Progress in Organic Coatings (2005), 54(3), 230-239. These disclosures suggest the combination of photoinitiated polymerization and crosslinking with a thermally-initiated radical polymerization, which is made possible by the inclusion of both a photoinitiator, such as an acylphosphine oxide, and a suitable redox thermal initiator, such as cerium(IV) ammonium nitrate [Ce(NH4)2(NO3)6], in the coating precursor material. Such a dual-cure process is said to be viable for automobile pigmented paint and clearcoat applications. For coatings on items such as an automobile body or portion thereof, the shape inherently causes UV illumination to be at least nonuniform, if not completely shadowed, in portions of the object. However, the dual-cure processes suggested by the Studer references require that the substrate be heated. In some of the examples given, a temperature of about 140° C. is specified. Many polymer substrates cannot withstand such a temperature. Although some curing would occur at lower temperatures, the kinetics of the cross-linking reaction would then dictate impractically long hold times. Thus, a process involving thermal curing is not even a feasible option for many substrate materials.
The shadowing problem is especially vexing in connection with the coating of generally planar but fibrous materials, in which substantial portions of the effective surface are shadowed by the inherent topology of the surface. Application of the coating precursor material, especially if done by vapor-phase methods, inevitably causes some of the precursor material to be deposited in interstices created by the network of fibers defining the surface layer. These interstices are below the bulk surface of the substrate, but are still in its immediate vicinity. They are readily able to communicate with the surrounding atmosphere. Directing beam radiation to impinge on the fibrous sheet material at varying angles of incidence only partially mitigates shadowing, because the inherent topology of the surface texture dictates that the underside of some fibers has no outward-facing exposure.
Planar, fibrous sheet materials used in the building construction industry as moisture vapor-permeable sheets for wall and roof wrapping provide an example in which the problem of shadowing can arise, as some forms of these materials include a surface polymeric coating that must be cured by cross-linking.
US Published Patent Application No. US2008/0187740 to Bletsos et al. (“the '740 publication”), which is commonly owned with the present application, discloses a metalized, moisture vapor permeable composite sheet formed by coating at least one side of a moisture vapor permeable substrate with at least one metal layer and at least one thin polymeric coating layer on the side of the metal layer opposite the substrate. The coating may be formed under vacuum using vapor deposition techniques under conditions that substantially coat the substrate without significantly reducing its moisture vapor permeability. The composite sheet is said to have high moisture vapor permeability, and good thermal barrier properties. The composite sheet can also be selected to provide a high barrier to intrusion by liquid water (signaled by a high hydrostatic head), which is another important characteristic for construction end uses such as house wrap and roof lining. Such a composite sheet is said to provide a thin, strong, breathable air and thermal barrier that is suitable for use in existing or new construction.
Notwithstanding these advances, there remains a need for improved products in which coated fibrous materials can be produced efficiently yet retain their desirable physical and structural properties throughout their entire lifecycle.